In order to receive the greatest return from the cost of
generating and using steam as a heating and process medium, a good working
knowledge of a steam traps purpose, operation, and application is
essential. The information contained in
this chapter is designed to aid you in effectively selecting steam traps since
most problems encountered by users can be prevented by installing the correct
type of trap with an approved hook up.
Why a steam trap?
Steam generated by a boiler contains heat units, British
Thermal Unit, which are released in the heating, process applications, and by
pipe radiation loss, and in doing so the steam returns to its water, or
condensate, state. If condensate is not
drained immediately or “trapped” from the system it reduces operating
efficiency by slowing the heat transfer process and can cause physical damage
through the phenomenon known as “Water Hammer.”
velocity of the steam passing over it sweeps a condensate accumulating on the
bottom of a horizontal pipe along. As
the velocity increases, the condensate can form a solid slug of incompressible
water, which, when it is suddenly stopped by a pipe bend, fitting, or valve,
produces a shock wave of tremendous momentary force, which can cause great
physical damage. Since water hammer is
a function of steam velocity, it is as dangerous in low pressure systems as it
is in high pressure systems and the piping must be designed to drain properly
avoiding water pockets.
trap is therefore required at all locations where condensate is formed and
Steam Distribution Mains:
At elevation changes
such as risers and expansion loops.
At all low points and
at intervals of 60 to 150 metre. On long horizontal runs.
A head of all
possible “dead-ended” areas such as shutoff valves, pressure and temperature
control valves, and at ends of mains.
Steam Tracing Service:
traced and jacked piping and valves.
For freeze protection
of instruments and equipment.
Steam Operated Equipment:
of humidifiers, pumps, turbines, etc.
below heat exchangers, coils, unit heaters, cooking kettles,
The air and other non-condensable gasses such as carbon
dioxide and carbon monoxide must also be rapidly purged from the steam system
by the trap or by an auxiliary air vent for four important reasons.
On start-up, steam
enters the system only as fast as the air is vented.
An air-steam mixture
has a temperature well below steam temperature lowering
the heat transferred.
Air is an insulator
and clings to the surface of the pipe or vessel causing slow and
condensate, these non-condensable gasses form corrosive carbonic
steam trap therefore is an automatic drain valve which must sense the
difference between steam and condensate, operate under varying inlet and back
pressures, changing condensate loads and must also release non-condensable
gases while not wasting any steam.
The temperature at which the condensate is discharged by
the trap is important in maintaining energy efficiency. While most applications require that the
condensate be discharged at close to steam temperature utilizing the steams
latent heat, some may tolerate some degree of water logging and thereby also
use the sensible heat contained in the condensate as it cools down to
100șC. The type of trap selected must
therefore be matched to its intended use if the most effective energy use of
the steam system is to be realized.
PRINCIPLES OF STEAM TRAP OPERATION
many different types of steam traps manufactured today all operate by sensing
the difference between steam (a hot gas) and condensate (a cooler liquid) using
one or more of three basic principles, each design has advantages and
limitations which must be considered when selecting a steam trap.
Density Operated—(Mechanical traps)
Kinetic Energy Operated—(Disc and
Thermo –Dynamic Disc
Impulse or Orifice
DENSITY OPERATED TRAPS
operated traps are purely mechanical in operation. They use some type of float Bucket to determine the condensate
level in the body and thereby operate a valve mechanism. Each float Bucket mechanism combination has
a fixed mechanical advantage requiring the valve seat orifice to be matched to
a maximum operating pressure.
Float and Thermostatic Traps:
and thermostatic traps have much to recommend them wherever possible. Their valve seat is always under water
preventing any steam loss. The discharge is continuous and modulates with the
condensing rate, and it is unaffected by any change in inlet pressure. A
separate thermostatic air vent independently purges air giving a fast start-up
and discharges in parallel with the main valve seat unaffecting its operation.
entering the trap is immediately discharged through a high capacity auxiliary
air vent. Condensate causes the ball
float to rise and place the modulating discharge valve in a position that will
pass the condensate continuously as it enters the trap. The condensate level in the trap body is
maintained above the discharge valve to provide a positive seal against the
loss of steam.
continuously as rapidly as it forms.
High air venting
capacity through auxiliary balanced pressure air vent, which is
for varying steam pressures.
efficiency at both light and heavy loads.
discharge does not create pressure disturbances, which
may cause erratic
control in air hearing coils, shell-and-tube exchangers, etc
Steam lock release
(S.L.R.) facility available.
Inline inlet and
outlet facility, easy installation at low cost.
resistance to water hammer.
Fail close, so no
wastage of steam energy.
Unaffected by changes
in inlet pressure.
Cannot be used where
trap is fitted with air vent bellows, & degree of super
heat > 100șC
subjected to freezing must be protected with insulation & SLR.
Water hammer can
damage float and air vent bellows.
unit heaters, Hot water heaters, Heat exchangers, converters, Reboilers,
Jacketed Pans etc.
bucket traps have been in existence for many years, and an inexpensive initial
cost helps keep them popular although in every application superior results can
be obtained with another type of trap.
They consume a small amount of steam in operation and can blow fully
open if they lose their prime due to over sizing or a rapid drop in inlet
pressure. Their discharge is
intermittent, not continuous.
bucket traps are variation, which are no longer generally accepted and are
marketed today as an open-float and thermostatic trap. This type of trap has no mechanical
advantage and therefore requires a large heavy bucket, which collects dirt and scale. They have large heavy bodies with limited
operating pressures and applications.
trap body is normally filled with condensate to maintain a seal around the
inverted bucket, which serves as a float to operate the discharge valve. Live
steam entering the bucket floats it to close the valve. During the closed
period, condensate collects in the piping at the inlet side of the valve until
steam floating the bucket leaks through a small hole in the top of the bucket
and permits the bucket to drop and open the valve. The condensate is discharged, followed by steam, which is
required to actuate the float mechanism. Air can pass through the small hole at
the top of the bucket. Some inverted
bucket traps are fitted with an auxiliary bimetal air vent.
Fair resistance to
Can be made for very
efficiency under varying loads and pressures, some steam loss for
Must maintain water
seal to avoid continuous discharge of steam.
Must be protected
condensate continuously as rapidly as it forms.
Bleed hole in bucket
has very limited air-venting capacity.
Bimetal auxiliary air
vent must be factory set at low temperature—not
pressure indoor steam main drips and submerged heating coils.
Traps of this type all sense the difference between steam
temperature and cooler condensate temperature utilizing an expanding bellows or
bimetal strip to operate a valve head.
They usually discharge condensate below steam temperature and some
models require a collecting leg before the trap to allow for condensate
Balanced-Pressure Thermostatic Traps:
Balanced-Pressure Thermostatic traps have a full-open
“Blast-type” discharge which permits fast startups and high air venting
capacity but limits their operation to pressure up to 17 BAR. Their valve seat is wide-open when cold
making them freeze-proof in outdoor use.
The bellows material may be brass, phosphor bronze, or corrosion
resistant monel or stainless steel.
Balanced Pressure Thermostatic Trap is sensitive to pressure variation, and
adjusts to discharge condensate at pre-determined temperature differential at
all pressures within its range.
Liquid Expansion Traps:
liquid expansion type traps sense the condensate temperature downstream from
the seat, which can be a maximum of 100șC and are therefore highly thermal
efficient because they use the sensible heat contained in the condensate. But due to sluggish response and non-self
adjusting characteristic, its use is limited.
Bimetal operated traps sub-cool the condensate well below
saturated steam temperature before it is discharged. This means that partial
flooding of the steam lines occurs ahead of the trap and care must be taken
when they are used to drain steam mains or water hammer damage may result. However, applications such as steam tracing
lines, which can tolerate some back up, can realize energy saving by reclaiming
some of the sensible heat content of the condensate as it cools down. The bimetals react rather slowly to load
changes and may require adjustment for tight shut off due to metal fatigue and
operating condition changes. This is not necessary with the Spirax Sarco SM21
trap because its patented “Multi-cross” bimetal element can self-adjust to
operating pressure changes.
Bimetallic traps perform well on superheated steam
applications when installed with an adequate cooling leg ahead of them.
The trap is operated by a flexible thermostatic bellows
filled with a fluid, which when heated or cooled, evaporates or condenses. Internal pressure changes expand or contract
the bellows and positions the attached valve head. On start up, the cold bellows is contracted, and the wide-open
valve remains open to discharge air and sub cooled condensate. When steam reaches the trap, the bellows
expands and closes the trap. When
condensate surrounding the bellows cools to approximately 8ș to 20șC below
steam temperature (depending on the filling), the trap opens, to discharge
air venting capacity for fast start-up.
capacity in small size.
will operate without adjustments at all pressures within its range.
not freeze if given free discharge.
same valve sizes for all pressures within its operating range.
in vacuum systems.
Steam radiators, Low and medium pressure submerged heating
coils, sterilizers, and steam tracer.
are not suitable for highly superheated steam.
resistance to water hammer ad corrosion depending on bellows construction.
suitable for applications in which condensate must be discharged as fast as it
is formed. Condensate must cool before
it can be discharged.
discharge at higher pressures with most models
LIQUID EXPANSION THERMOSTATIC TRAP
Air and condensate are discharged on start-up until he
condensate reaches a predetermined temperature below 100șC. The liquid-filled thermostatic element then
throttles the valve to maintain the pre-set condensate discharge temperature.
high thermal efficiency (utilizes sensible heat along with latent heat of
discharge eliminates flash steam around operating stations.
not freeze when given discharge
fails in an open position.
tracing lines, Storage tank coils, and open tank heating coils.
to applications such as storage tanks and some tracer lines where condensate
can be held back and sub cooled before discharge.
condensates can attack bronze bellows in thermostatic element.
self-adjusting to pressure changes.
have an open discharge outlet.
BIMETALLIC THERMOSTATIC TRAPS:
Air and condensate are discharged on start-up until the
condensate reaches a predetermined temperature. The bimetal thermostatic
element then throttles to maintain the preset sub-cooled condensate discharge
temperature and some flooding of the steam line may occur. Tight shutoff depends on accurate adjustment
of the valve stem length unless the bimetals are designed to self-adjust to
high thermal efficiency when set to discharge at low temperature.
discharge eliminates flash steam around operating stations.
designs will not freeze when given free discharge.
well on superheated steam applications.
Steam tracing lines that can tolerate partial flooding,
storage tank coils.
operation and slower response
to applications in which condensate can be held back and sub cooled before
characteristics may change after being in use requiring service.
self-adjusting to inlet pressure changes.
Traps that operate on kinetic energy use the velocity and
phase change of pressurized condensate flashing to lower pressure steam as the
moving force to operate a valve. They
appear to be of more simple construction than any other type of trap but
successful operation depends on a higher degree of engineering design and skill
than with any other type of trap.
Disc traps are the most widely used steam traps today
largely due to their small size, wide pressure range, one moving part, and
resistance to water hammer and corrosion.
Because operation of each model depends on the manufacturers seat and
disc design, results obtained by the user may vary widely. Many are prone to air-binding on start-up,
operate below steam temperature causing water logging, have a relatively short
life due to soft seat and disc materials, and contain a bleed slot which causes
rapid cycling and steam loss.
The Spirax Thermo-Dynamicź series of traps have been
carefully engineered to overcome all of the above problems. They discharge condensate and entrapped air
at steam temperature regardless of the inlet pressure—with no steam loss. Their hardened seat and disc insure a long
service life, even with high backpressures, and the audible cycle provides an
instant test of its operation.
The first operational design was the impulse trap in which
two orifices in series create a changing intermediate pressure to operate a
piston-type of valve head. It operates
far below steam temperature backing up considerable amounts of condensate before
opening. Because it requires a constant
bleed for operation, it can waste steam on light loads and its use has largely
been displaced by disc traps.
As hot condensate passes through an orifice, the amount of
downstream flash produces a backpressure, which increases with the condensate
temperature and thereby throttles somewhat the flow rate through the
orifice. Orifice devices, which are
sold to replace steam traps, must therefore be accurately sized to a constant
load and inlet pressure, which prohibits any variation between start-up and
operating conditions, or any safety factor.
Since orifice devices cannot automatically adjust to any changes they
should not be confused with, or used in place of a steam trap. Their small size makes them prone to both
erosion and clogging, and light loads and pressure changes can produce live
Condensate and air raise the disc and flow freely through
the trap. When steam reaches the trap,
the velocity under the disc is instantly increased, and recompression above the
disc snaps it onto its seats to give a tight shut-off. Heat loss from the small control chamber,
which is filled with a steam/condensate mixture, causes the chamber pressure to
decrease to a point at which the valve disc opens again to discharge
stainless steel construction for good corrosion resistance.
resistance to water hammer.
life due to hardened seat and disc.
one moving part.
trap for all pressures from 0.6 to 100 BAR
operation under varying loads and pressures.
response to changing loads.
condensate at steam temperature, to prevent water logging.
Steam main drips, High pressure and superheat, Steam
tracer lines, Unit heaters.
suitable for pressures below 0.6 BAR
models limited to 50% backpressures, others suitable for 80% backpressures.
recommended for low-pressure applications with temperature control valves.
Uses two orifices in series to provide a pressure impulse
to operate the discharge valve. When
relatively cool condensate reaches the trap, it passes through the two orifices
in series without creating sufficient pressure in the control chamber to close
the main valve. Condensate continues to
flow until it reaches a temperature approximately 17șC below steam temperature,
at which point pressure in the control chamber can close the trap. As the
condensate is held back and is cooled, the trap again opens, and the cycle is
repeated. Under light loads, live steam
will reach the trap and be discharged through the bleed orifices.
stainless steel construction.
resistance to superheat and water hammer.
bleed orifices will waste steam on light loads.
valve parts are subject to sticking.
held back can cause water logging.
held back may contribute to corrosion and water hammer.
not be used where backpressure will exceed 30% of inlet pressure.